1. Field of the Invention
The present invention concerns a method for diffusion-weighted acquisition of MR signals with an acquisition sequence that includes multiple diffusion coding gradients, and a magnetic resonance system for implementing such a method.
2. Description of the Prior Art
Diffusion-weighted magnetic resonance (MR) images in the clinical routine can deliver important diagnostic information, for example in stroke and tumor diagnostics. In diffusion-weighted imaging (DWI), diffusion coding gradients with high amplitude and long pulse duration are used in combination with a suitable readout or image data acquisition module. The sensitivity to the diffusion follows from the fact that the diffusion of water molecules reduces along the applied field gradients of the magnetic resonance (MR) signal. In regions of an examined subject with low diffusion, a lower signal loss thus occurs, and the regions are accordingly imaged with a higher signal intensity. The strength of the diffusion weighting is correlated with the strength of the diffusion coding gradients. The diffusion weighting is characterized by what is known as the b-value, which is a function of gradient parameters (for example the gradient strength, duration or the interval between gradients). To avoid movement artifacts, a “single shot” readout module can be used, for example, with which a complete raw data set is acquired during a single acquisition sequence. For example, an echoplanar imaging sequence (EPI) can be used.
The most prevalent method for diffusion coding uses the monopolar spin echo acquisition sequence described by Stejskal and Tanner (see “Spin Diffusion Measurements Spin Echoes in the Presence of a Time-Dependent Field Gradient” in The Journal of Chemical Physics 42, 1965). In this method, two strongly symmetrical gradients are positioned on each side of a 180° refocusing pulse in a spin echo sequence. These symmetrical diffusion coding gradients have the purpose of accelerating the signal loss caused by the diffusion by promoting the dephasing. The dephasing is normally proportional to the square of the time during which the gradient is switched (activated) (gradient pulse length), to the time interval of the two gradient pulses and to the square of the strength of the switched gradient field.
The signal-to-noise ratio (SNR) and geometric distortions are crucial to the quality of acquired images. Furthermore, the gradient systems that are used limit the maximum switchable strength of the magnetic field gradients. The imaging parameter that is relevant to the signal-to-noise ratio is the echo time TE. The geometric distortions are based on spatial variations of the basic field amplitude B0, wherein EPI is especially sensitive to this. Static distortions are caused by the basic field inhomogeneity and the susceptibility of different regions of the examination subject. Dynamic distortions (for example eddy current effects) are in particular affected by the temporal sequence of the gradient pulses. Every activation and deactivation of field gradients can cause such eddy currents that decay with different time constants. With a slow decay, field components can remain until the readout, such that disruptions and distortions of the acquired MR data can result. Particularly in the combination of images acquired with different diffusion gradient directions and amplitudes, it is desirable to keep the dynamic distortions as small as possible in order to reduce errors in the resulting data (for example anisotropy maps, diffusion maps, tensor data and the like).
The monopolar spin echo acquisition sequence described by Stejskal and Tanner exhibits strong geometric distortions, in particular a high proportion of residual eddy current fields, and a high loading of the gradient system. To avoid the strong distortions, further-developed acquisition sequences use bipolar double spin echo schemes that can implicitly compensate the eddy current fields. Such a scheme is described by, for example, T. G. Reese et al.: Magnetic Resonance in Medicine 49:177 (2003). Disadvantages of the monopolar scheme can be reduced with such a scheme, but at the cost of a longer echo time. For example, the additional radio-frequency (RF) pulse that is to be used requires 5 ms. Due to relaxation time properties of the examined tissue, the longer echo time can disadvantageously affect the image quality. Furthermore, additional unwanted signal coherence paths are introduced by the additional RF pulses, which can lead to additional unwanted spin echo signals or stimulated echo signals as well as to free induction decay signals. These additional coherence paths cause artifacts in the image data that are reconstructed from the acquired MR data. Spoiler gradients are known for suppression of such unwanted coherence paths. However, such additional gradients in turn extend the echo time (TW) and thus disadvantageously affect the signal-to-noise ratio. A more complicated echo is also necessary with these.
It is desirable to efficiently suppress the image artifacts caused by the additional signal coherence paths. The echo time should not be extended by this in order to achieve a good signal-to-noise ratio. It is also desirable to keep the distortions low given an optimal utilization of the gradient system.